Techniques for detecting the number or the volume of particles by using the impedance of a sheath flow (known as “sheath flow impedance method”) may be dated back to U.S. Pat. No. 3,793,587 and No. 3,810,010 in 1974. These patents involve some common elements, such as a flow cell including a front chamber and a back chamber, an aperture (or a small through hole), a sample tube, electrodes and a detection circuit. Here, the front and back chambers are connected via the aperture, with which the specimen tube is disposed coaxially. The outlet of the specimen tube is located at a position in the front chamber near the aperture, so as to introduce the particle suspension liquid to be detected (referred to as liquid specimen hereafter) into the flow cell. Each of the front and back chambers has a conductive particle free liquid source, hereafter referred to as front sheath flow and back sheath flow, respectively. The pressure of the front sheath flow is equal to the pressure of the liquid specimen at the outlet of the specimen tube; while the pressure of the back sheath flow may be much lower than the pressures of the former two.
In the counting circuit based on the sheath flow impedance, an electrode connected to the high-potential has to be placed in one of the front and back chambers, which is abbreviated as high-potential chamber; and a zero-potential electrode is placed in the other chamber, which is abbreviated as zero-potential chamber. To pass all the currents from the high-potential to the zero-potential through the aperture of the flow cell and ensure that interference will not be introduced or the sensitivity of the sensor will not be reduced due to the existence of other branch currents, it is preferable to isolate the high-potential chamber from the upstream of all liquid passages that supply conductive liquid to it, so as to avoid the possibility that branch currents are formed by connecting the upstream of these liquids to the zero-potential.
In U.S. Pat. No. 4,070,617 and No. 4,198,160, sheath liquid driving is implemented in the manner of driving by the liquid level difference. In U.S. Pat. No. 4,165,484, an exterior pressure source is introduced, in which a common air supply drives the sheath liquid in the storing chamber, and the liquid in the sheath liquid storing chamber is driven under the pressure to provide sheath liquid to the flow cell. To reduce the interference brought from the liquid storing chamber to the counting circuit, the inventors of U.S. Pat. No. 4,165,484 proposes a method of serially connecting a glass capillary tube within the sheath liquid supply passage, so as to increase the resistance of the liquid passage, and thus to achieve an effect of electromagnetic isolation. In this way, however, the liquid passage from the sheath liquid storing chamber to the flow cell plays a role of a conductive line unavoidably, thus introducing electromagnetic interfering signals into the flow cell. The signals will be superimposed on the impedance signals of the aperture, which makes it harder to identify signals. To decrease interferences, the glass capillary tube should be long and slim, which increases the operational risks. Meanwhile, to decrease interferences and avoid introducing additional electromagnetic interferences, electromagnetic valve, which is smaller and easily controllable, should be avoided to control the liquid passage. Furthermore, the driving pressure of the back sheath flow is larger in this method, thus causing increased consumption of liquid.
In U.S. Pat. No. 5,905,214, a positive pressure is applied and the driving force for ejecting the back sheath liquid into the back chamber is the positive pressure from the upstream chamber. To ensure that the positive pressure can be transmitted to the back chamber, the liquid passage along the way has to be sealed very nicely and a continuous liquid supply is a must. Consequently, it would be difficult to prevent exterior exoteric interfering signals from entering into the back chamber. In U.S. Pat. No. 6,909,269, the flow cell is placed within a special shielding box, which may attain shielding effect to some extent but cannot prevent occurrence of back-flow of the liquid specimen and generation of fake signals after the liquid specimen enters into the back chamber.